1. Field of the Invention
The present invention is related to an electron gun for use in a color cathode ray tube or high definition industrial picture tube, and more particularly, to a structure of a focusing electrode in an electron gun for a color cathode ray tube, in which a gap between first and second focusing electrodes can be arranged closer for improving an STC(Static Convergence Drift) occurred during operation of the electron gun.
2. Discussion of the Related Art
The electron gun is a device in which three electron beams emitted from cathodes are focused on a fluorescent screen consisting of red, green and blue fluorescent materials coated on an inside surface of the cathode ray tube. Each of the fluorescent materials reacts with one of the electron beams to emit a fluorescent light with, a combination of the three beams forming a pixel.
FIG. 1 illustrates a cross section of a general in-line type electron gun.
Referring to FIG. 1, the electron gun 1 includes a triode 2 and main focusing static lens 3. The triode 2 has cathodes 4 each for emitting thermal electrons toward a screen, a control electrode 5 for controlling the thermal electrons, and an accelerating electrode 6 for accelerating the thermal electrons, arranged in the aforementioned order. The main focusing static lens 3 arranged in front of triode 2 has a focusing electrode 7 and an anode 8. When voltages of preset levels different from one another are applied to the different electrodes respectively, the electron beams are controlled and focused into intended intensities by the controlling electrode 5 and the accelerating electrode 6, focused by a main focusing static lens formed between the focusing electrode 7 and the anode 8, and accelerated by the anode 8 toward the screen. Then, the electron beams are deflected by a non-uniform magnetic field formed by deflection yokes to make a self convergence, and form a pixelon the screen. However, the application of the non-uniform magnetic field causes the electron beams to form a horizontally elongated spot together with haze. Haze is a thinning of an image on upper and lower sides of the horizontally elongated spot caused by a synergy effect of the focusing power of the magnetic field which is weak in horizontal the direction and strong in the vertical direction. The haze can be eliminated or reduced by forming a well known dynamic four polar correcting lens between a divided focusing lens upon application of a voltage synchronous to a deflection signal to one of the divided focusing lens.
FIG. 2 illustrates a perspective view of a disassembled conventional focusing lens divided into two to form the dynamic four polar correcting lens.
Referring to FIG. 2, the focusing electrode 7 includes a first focusing electrode 71 adapted to be applied of a static voltage, a second focusing electrode 72 arranged next to the first focusing electrode 71 and adapted to be applied of a dynamic voltage for producing a voltage difference higher than the voltage to the first focusing lens 71 in a range of 300.about.1000 V depending on extent of deflection of the electron beams, electron beam pass through holes 711 and 721 formed in the first and second focusing lenses 71 and 72 at facing end surfaces 712 and 722, and a pair of burring parts 723 at an upper and a lower portions of each of the electron beam pass through holes 721 in the second focusing electrode 72 projected toward or inserted in one of the electron beam pass through holes 711 in the first focusing electrode 71. With this configuration, when the electron beams are deflected, the second focusing electrode 72 is applied of the dynamic voltage to form the dynamic four polar correcting lens between the first and second focusing lenses 71 and 72 by the voltage difference formed between them. The burring parts 723 provided at the upper and lower portions of the electron beam pass through holes 721 in the second focusing electrode 72 permitting the dynamic four polar correcting lens to correct the horizontal elongation of the electron beam spot. However, as shown in FIG. 3 A, during formation of the burring parts 723, stresses are generated at a circumference of the electron beam pass through holes 721 where the burring parts 723 are not provided, resulting in cracks 724 therein, that reduce the performance of the electron beams. FIG. 3B illustrates low burring parts 723L provided at horizontal portions of the electron beam pass through holes 721 for preventing generation of the cracks at the circumference of the electron beam pass through holes 721 in the second focusing electrode 72. Because of the added length of low burring parts 723L, the length of the high burring part 723H is lengthened as much as the length of the low burring part 723L to offset the influence from the low burring part 723L, which causes a gap D between the first and second focusing electrodes 71 and 72 to become greater as much as the length of the low burring part 723L as shown in FIG. 3C. The gap D between the first and second focusing electrodes 71 and 72 should be maintained to be in a range of 0.5 mm.about.0.6 mm. If the gap D is smaller than 0.5 mm, discharges can occur, and if the gap is greater than 0.6 mm, an STC drift, in which variation of focusing of the electron beams takes place as time changes may occur. According to a test room experiment, if the gap D between the first and second focusing electrodes 71 and 72 is greater than 0.8 mm, the electron beams are affected negatively. When the low burring part 723L is provided, the gap D between the first and second focusing electrodes 71 and 72 is in general greater than 0.8 mm.